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<div class="title">boost::hana::common&lt; T, U, enabler &gt; Struct Template Reference<div class="ingroups"><a class="el" href="group__group-core.html">Core</a></div></div>  </div>
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<a name="details" id="details"></a><h2 class="groupheader">Description</h2>
<div class="textblock"><h3>template&lt;typename T, typename U, optional when-based enabler&gt;<br />
struct boost::hana::common&lt; T, U, enabler &gt;</h3>

<p>Metafunction returning the common data type between two data types. </p>
<p><code>common</code> is a natural extension of the <code>std::common_type</code> metafunction to data types. Given two data types <code>T</code> and <code>U</code>, we say that they share a common type <code>C</code> if both objects of data type <code>T</code> and objects of data type <code>U</code> may be converted (using <code>to</code>) to an object of data type <code>C</code>, and if that conversion is equality preserving. In other words, this means that for any objects <code>t1, t2</code> of data type <code>T</code> and <code>u1, u2</code> of data type <code>U</code>, the following law is satisfied: </p><div class="fragment"><div class="line">to&lt;C&gt;(t1) == to&lt;C&gt;(t2)  <span class="keywordflow">if</span> and only <span class="keywordflow">if</span>  t1 == t2</div>
<div class="line">to&lt;C&gt;(u1) == to&lt;C&gt;(u2)  <span class="keywordflow">if</span> and only <span class="keywordflow">if</span>  u1 == u2</div>
</div><!-- fragment --><p>The role of <code>common</code> is to provide an alias to such a <code>C</code> if it exists. In other words, if <code>T</code> and <code>U</code> have a common data type <code>C</code>, <code>common&lt;T, U&gt;::type</code> is an alias to <code>C</code>. Otherwise, <code>common&lt;T, U&gt;</code> has no nested <code>type</code> and can be used in dependent contexts to exploit SFINAE. By default, the exact steps followed by <code>common</code> to determine the common type <code>C</code> of <code>T</code> and <code>U</code> are</p><ol type="1">
<li>If <code>T</code> and <code>U</code> are the same, then <code>C</code> is <code>T</code>.</li>
<li>Otherwise, if <code>true ? std::declval&lt;T&gt;() : std::declval&lt;U&gt;()</code> is well-formed, then <code>C</code> is the type of this expression after using <code>std::decay</code> on it. This is exactly the type that would have been returned by <code>std::common_type</code>, except that custom specializations of <code>std::common_type</code> are not taken into account.</li>
<li>Otherwise, no common data type is detected and <code>common&lt;T, U&gt;</code> does not have a nested <code>type</code> alias, unless it is specialized explicitly.</li>
</ol>
<p>As point 3 suggests, it is also possible (and sometimes necessary) to specialize <code>common</code> in the <code><a class="el" href="namespaceboost_1_1hana.html" title="Namespace containing everything in the library.">boost::hana</a></code> namespace for pairs of custom data types when the default behavior of <code>common</code> is not sufficient. Note that <code>when</code>-based specialization is supported when specializing <code>common</code> in the <code><a class="el" href="namespaceboost_1_1hana.html" title="Namespace containing everything in the library.">boost::hana</a></code> namespace.</p>
<blockquote class="doxtable">
<p>#### Rationale for requiring the conversion to be equality-preserving This decision is aligned with a proposed concept design for the standard library (<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3351.pdf">N3351</a>). Also, if we did not require this, then all data types would trivially share the common data type <code>void</code>, since all objects can be converted to it. </p>
</blockquote>
<h2><a class="anchor" id="autotoc_md181"></a>
Example</h2>
<div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div>
<div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div>
<div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div>
<div class="line"> </div>
<div class="line"><span class="preprocessor">#include &lt;<a class="code" href="core_2common_8hpp.html">boost/hana/core/common.hpp</a>&gt;</span></div>
<div class="line"> </div>
<div class="line"><span class="preprocessor">#include &lt;type_traits&gt;</span></div>
<div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div>
<div class="line"> </div>
<div class="line"> </div>
<div class="line"><span class="keyword">struct </span>Person { };</div>
<div class="line"><span class="keyword">struct </span>Employee : Person { };</div>
<div class="line"> </div>
<div class="line">static_assert(std::is_same&lt;hana::common&lt;int, float&gt;::type, <span class="keywordtype">float</span>&gt;{}, <span class="stringliteral">&quot;&quot;</span>);</div>
<div class="line">static_assert(std::is_same&lt;hana::common&lt;Person, Employee&gt;::type, Person&gt;{}, <span class="stringliteral">&quot;&quot;</span>);</div>
<div class="line"> </div>
<div class="line"><span class="keywordtype">int</span> main() { }</div>
<div class="ttc" id="acore_2common_8hpp_html"><div class="ttname"><a href="core_2common_8hpp.html">common.hpp</a></div><div class="ttdoc">Defines boost::hana::common and boost::hana::common_t.</div></div>
<div class="ttc" id="anamespaceboost_1_1hana_html"><div class="ttname"><a href="namespaceboost_1_1hana.html">boost::hana</a></div><div class="ttdoc">Namespace containing everything in the library.</div><div class="ttdef"><b>Definition:</b> accessors.hpp:20</div></div>
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